Conventional spark ignition, four stroke, internal combustion engines generally operate with intake and exhaust valves in each cylinder actuated with a fixed timing relative to an engine crankshaft, and with a throttle valve upstream of the intake valves to control the engine load by throttling the air flow. Further, the compression ratio of each cylinder is fixed regardless of engine speed. This basic configuration, consequently, has drawbacks that limit the efficiency of the engine for most operating conditions, which others have tried to overcome.
One drawback is that the load is controlled by throttling the air past the throttle valve in order to operate at other than full load conditions. This, along with other restrictions of the air flow, creates pumping losses in the engine. One attempt to overcome the pumping losses is to provide for variably actuated intake and exhaust valves that allow for control of the intake flow, and hence load control, by varying the opening and closing of the valves rather than employing a throttle valve. This does reduce the pumping losses and gives greater control of engine operation over a wider range of engine operating conditions. However, this solution is limited by the fact that controlling the intake of the air/fuel mixture at low load and idle conditions will substantially reduce the compression ratio at those conditions, resulting in poor combustion characteristics for these load conditions. One solution, then, is to increase the compression ratio for each cylinder in the engine at low load conditions. One such attempt has been to provide a variable compression ratio assembly where adjustments are made to the piston travel or the cylinder volume to allow for an overall higher compression ratio, while at high loads reducing the compression ratio to avoid knock. These solutions, however, add great cost and complexity to the overall engine. Further, one of the main disadvantages of spark ignition engines relative to other known engines, such as diesel, are their inability to operate with a high compression ratio. Generally, spark ignition engines (Otto cycle engines) with higher compression ratios are more efficient, thus improving fuel economy. To prevent knock in a spark ignition engine, its compression ratio must be restricted to much lower values than those used in other engines such as diesels, and this is an important factor contributing to its lower fuel efficiency relative to diesel. Thus, by permitting the spark ignition engine to operate with a high compression ratio without knock, it can substantially increase the engine efficiency.
Generally, then, a fixed compression ratio is the economical way to design an engine. For an engine, nonetheless, the compression ratio has to be held below an upper limit at heavy load conditions to prevent knocking. Consequently, it operates at this low compression ratio condition even for low and medium load conditions where knocking is not a concern and a higher compression ratio would operate more efficiently. Further, this limits the ability to operate without throttling the air flow.
The reason for the knock at full-load conditions is that the amount of combustion generated heat is great, which in turn, leads to an excessive temperature of the unburned gas fraction in the cylinder and hence the onset of knock. This puts a limit on the total amount of heat that can be transferred to the cylinder charge at a given engine speed during compression and combustion. At part-load, when the amount of combustion heat is relatively small, the engine could run knock-free with high compression ratio.
A further attempt to overcome the obstacles to maximizing engine efficiency relates to the flow characteristics for the air/fuel entering the cylinder. Less than ideal fluid flow and mixing will result in less than complete combustion, slow flame speed and burn rate, or both. Turbulence has been recognized as a primary driving force for good combustion in spark ignition engines. The practical and effective ways to increase turbulence production during combustion processes have been through the swirl- and tumble-generation devices. Split (dual) intake ports is an example of swirl-generating devices and tumble port used in four-valve engines is an example of tumble-generating device. Some have attempted to improve the flow characteristics by the geometry of the intake ports, employing vanes, or masking in order to improve mixing and create a swirl or tumble flow within the cylinder. However, these types of arrangements generally create flow restrictions, thus restricting and consequently limiting the maximum amount of flow into the cylinder and thus limiting the maximum engine output. Further, they are expensive to develop and produce. Thus, an inexpensive and reliable means for creating turbulence is desired that does not create flow restrictions.
Attempts have been made to overcome some of the above noted obstacles to maximum engine efficiency, but none satisfactorily improves most or all of these limitations of the conventional four stroke, spark ignition engine.